1. Field of the Invention
The present invention relates to methods and systems that detect gamma rays and reconstruct images, and particularly a technical solution that is applied to medical positron emission tomography (PET) equipment to acquire an image of a target position of a human body.
2. Description of the Related Art
PET is an important modern medical imaging technology used in the diagnosis of many diseases such as tumor and Alzheimer disease. A detected object is traced by a radioactive isotope (such as F-18) of β+ decay. β+ particles (also known positrons) emitted by the isotope collide with electrons within the detected object, causing annihilation and producing a pair of 511 keV gamma photons traveling along approximately opposite directions. Current clinical PET scanners are equipped with a detector system deployed in the form of a cylindrical array. In general, if the detector system detects a pair of 511 keV gamma photons within a very short time window by using a time coupling circuit, the system considers an annihilation event occurred somewhere on the line connecting the pair of gamma photons. This process is called time coupling, and also referred to as electronic collimation.
Objectively, the time coupling technology eliminates the need for collimation devices, improving the efficiency in counting of individual gamma photons. However, if multiple annihilation events occur within a short period of time and more than two 511 keV gamma photons are detected within the short time window, the system cannot determine in which line the event occurred, causing errors. If only one gamma photon is detected, there is no way to determine its traveling path, and thus this photon count can not be used for reconstruction. Additionally, time coupling usually requires using layers of detectors arranged in a ring shape, and uses a scintillator with a short light emission decay time, such as lutetium oxyorthosilicate (LSO), and fast-responsive electronic circuits and photomultiplier tubes (PMTS), pushing high the cost of PET systems. More importantly, the detectors in PET provide only a small spatial coverage, but annihilation photons could travel in any directions, resulting in a very low effective coupling ratio, i.e. when one detector detects a 511 keV gamma photon, the chance of one and only one other detector detecting a 511 keV gamma photon within timing-window is very low. That is because there is a high probability that the other gamma photon (generated from the same annihilation event) flies out of the area which the detector system covers, or the other gamma photon passes through a detector but is not detected due to the limited detecting efficiency. It has been reported that the effective coupled count is only 1-2% of the individual gamma photon count. This number includes some random coupling.
In astronomy, the coded aperture imaging technology is applied, which detects gamma rays emitted from celestial bodies using a coded aperture mask. A coded aperture collimation device is disposed between a detector and the measured space while collecting images, and the collimation device is parallel with the detector array. Gamma photons emitted from a point-shape celestial body travel through small openings of the coded aperture mask, and project onto the surface of detector array, forming a shadow matching the pattern of the small openings on the coded aperture mask. Because the distance from celestial body to the coded aperture mask is extremely long, the gamma photons emitted from the same celestial body are considered traveling in the same direction, namely the far field imaging. It is generally deemed impossible to apply this technology into near filed imaging such as medical imaging.